Optical unit and method for assembling the same

ABSTRACT

An optical unit includes a lens and a lens frame to support the lens. The lens frame includes a cylindrical portion and a U-shaped portion. The U-shaped portion has a U-shaped cross-section. The cylindrical portion has an edge and an inner surface in contact with an outer surface of the lens. The U-shaped portion is formed integrally with the edge of the cylindrical portion and holds the lens. With the lens held, the U-shaped portion is adhesively bonded to the cylindrical portion and then is removed from the cylindrical portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Applications No. 2001-209320, filed Jul. 10,2001; and No. 2001-209321, filed Jul. 10, 2001, the entire contents ofboth of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical unit including ahigh-precision objective unit having a lens, a lens frame, and a lensbarrel and a method of assembling the optical unit.

Further, the present invention relates to an optical unit having alens-holding apparatus suitable for a high-precision objective lens,etc.

2. Description of the Related Art

Conventionally, as shown in FIG. 7, a microscope objective unit includesa plurality of lenses 103 a through 103 f, a plurality of lens frames102 a through 102 d for holding the lenses 103 a through 103 f, and alens barrel 101.

The lens frames 102 a through 102 d have substantially the same outsidediameter. The lens frames 102 a through 102 d hold lenses 103 a through103 f. The lens frames 102 a through 102 d have center axes for theoutside diameters. The center axes of the lens frames 102 a through 102d substantially correspond to optical axes of the lenses 103 a through103 f.

The lens barrel 101 has the center axis. The lens barrel 101 iscylindrical and has a hole 101 a along the center axis. The hole 101 ahas an inside diameter substantially the same as the outside diameter ofthe lens frames 102 a through 102 d.

The lens frames 102 a through 102 d holding the lenses 103 a through 103f are inserted into the hole 101 a. The center axes of the lens framessubstantially correspond to the center axis of the lens barrel 101.Consequently, optical axes of the lenses 103 a through 103 fsubstantially correspond to each other in the hole 101 a.

The objective unit is assembled by inserting the lens frames holding thelenses into the lens barrel. This assembling method can providecentering more accurately than a method of assembling the objective unitby directly dropping the sets of lenses 103 a through 103 f into thelens barrel 101.

There is provided a plurality of types of lens frames as means forholding lenses. For example, the holding means is available as a lensframe as shown in FIG. 8. FIG. 8 shows an optical unit including theabove-mentioned lens frame. The optical unit has a set of lensesincluding lenses 103 g and 103 h. In the set of lenses, optical axes ofthe lenses 103 g and 103 h are coaxially arranged. In FIG. 8, thereference numeral 105 represents an optical axis of the set of lenses.

A lens frame 102 e has a predetermined outside diameter. The lens frame102 e has a center axis for the outside diameter. The lens frame 102 ehas a lens frame edge 104 orthogonal to the center axis. The lens frameedge 104 is formed with high precision along a direction orthogonal tothe center axis. The lens frame 102 e has a stop which extends towardthe inside of the frame itself along a direction orthogonal to thecenter axis. The stop has a retaining surface for seating a lens.

When the lens frame 102 e is used to hold lenses, a set of lenses(cemented lens) including the lenses 103 g and 103 h is dropped onto theretaining surface in the lens frame 102 e. The set of lenses is held onthe retaining surface of the lens frame 102 e. The set of lenses isfixed to the retaining surface with adhesive.

When adhesively bonding the above-mentioned lens set to the lens frame102 e, the lens 103 h is first placed on the retaining surface. The lensframe 102 e is then moved along the lens frame edge 104. Morespecifically, the lens frame 102 e rotates around its center axis on thelens frame edge 104. The lens 103 h is centered with respect to the lensframe 102 e by the rotating. The lens frame 103 h is then adhesivelybonded to the lens frame 102 e. Likewise, the lens 103 g is arranged onthe lens 103 h and is centered with respect to the lens frame 102 e. Thelens 103 g is then adhesively bonded to the lens 103 h. The lens frame102 e having the retaining surface is often used as a lens holding meansfor ordinary objective units.

FIG. 9 shows a lens frame 102 f as another example of theabove-mentioned holding means. The lens frame 102 f in FIG. 9 isconfigured to be capable of plastic deformation. The lens frame 102 f isconfigured to include a lens 103 i. The lens frame 102 f is caulked atits end to fix the lens 103 i. Accordingly, the lens frame 102 f can fixthe lens 103 i without using adhesive. The means for caulking the lensframe 102 f has been long used.

FIG. 10 shows a holding means capable of maintaining a clearance alongoptical axes of two lenses. FIG. 10 also shows an optical unit includingthe above-mentioned holding means. The optical unit has a set of lensesA comprising a convex lens 31 and a concave lens 32 bonded to each otherand a set of lenses B comprising a convex lens 33 and a concave lens 34bonded to each other.

The holding means in FIG. 10 has a lens frame 144 for holding the set oflenses A and a lens frame 146 for holding the set of lenses B. Theholding means further has a clearance ring 145 arranged between the lensframes 144 and 146. The clearance ring separates the lens frames 144 and146 with a predetermined clearance along center axes of the lens frames144 and 146. Consequently, the sets of lenses A and B can separatepredetermined clearance along the optical axes thereof.

The set of lenses in FIG. 8 including the lenses 103 g and 103 h isdropped into the lens frame 102 e to be seated in the lens frame 102 e.Accordingly, the lens frame 102 e can be bonded while the set of lensesis centered. The lens frame 102 e needs to have a stop because the setof lenses is dropped thereinto. The space for providing the stop isneeded around the lens frame 102 e.

As shown in FIG. 11, however, there is no space around the lens frame102 e for an optical system in which lenses are arranged close to eachother. It is difficult for the lens frame 102 e to maintain or bond theoptical system in FIG. 11.

The lens frame 102 f in FIG. 9 holds the lens 103 i by caulking asmentioned above. Generally, the lens frame 103 f is caulked at a verysmall portion thereof. Thus this caulking is difficult. When the lens103 i is fixed by caulking, the accuracy of centering depends on theworker's experience and skill. When optical unit are manufacturedthrough the use of caulking, there is a problem of widely varying thequality of the optical unit.

As mentioned above, the holding means in FIG. 10 has a clearance ring145 in addition to the lens frames 144 and 146. Since the holding meanshas a plurality of members, the structure is complicated. Accordingly,it is difficult to improve the accuracy of centering for the set oflenses A and B. Since the holding means has the complicated structure, aclearance between the set of lenses may not be provided highlyprecisely.

As shown in FIG. 12, there is devised a holding means having asimplified structure. The simplified structure is provided byintegrating the lens frame 144 and the clearance ring 145 in FIG. 10. InFIG. 12, the reference numeral 148 represents a lens frame formed byintegrating the lens frame 144 and the clearance ring 145. The use ofthe lens frame 148 simplifies the optical unit configuration. However,the lens frame 148 has a larger dimension in the direction along itscenter axis than that of the lens frame 144. Accordingly, the set oflenses A is arranged at an inner part in the direction along the centeraxis of the lens frame 148. The lens frame 148 makes it difficult tocenter the set of lenses A.

Especially, a microscope objective unit using wavelengths in anultraviolet range uses more lenses than a microscope objective unitusing wavelengths in a visible range for the following reason.

An ordinary glass does not transmit the light having a wavelength of 300nm or less. Accordingly, lenses for the ultraviolet range have alimitation on the use of a glass material such as fluorite or quartzwhich can transmit a wavelength of 300 nm or less.

Therefore, an objective unit for the ultraviolet range isdisadvantageous with respect to the correction of chromatic aberration.

In order to conduct the correction of chromatic aberration, it isnecessary to arrange objective unit lenses adjacently to each other. Onemethod of arranging the objective unit lenses adjacently to each otheris to use an adhesive to bond these lenses to each other. It should benoted that the ultraviolet light degrades the adhesive. Since thetransmittance of the bonded lenses decreases, it is undesirable to usean adhesive.

For the correction of chromatic aberration, it is therefore preferablethat the lenses are separated from each other with a predeterminedinterval. Widening an interval between lenses decreases the effect ofthe correction of chromatic aberration. It is desirable to adjacentlyarrange convex and concave lenses having different medium.

For the reason as mentioned above, the lenses are very closely arrangedin the microscope objective lens unit using wavelengths in theultraviolet range as shown in FIG. 11. In surfaces of the lenses facingto each other in FIG. 11, curvature radius Rp of the convex lensapproximately equals curvature radius Rn of the concave lens. In orderto provide substantially the same curvature radius, Rp/Rn must be set tosatisfy the following condition.

(Condition) 0.58<Rp/Rn<1.65

When the curvature radius of each facing surface satisfies thiscondition, the objective unit comprising the above-mentioned lenses canappropriately correct aberrations including the chromatic aberration.When the above-mentioned Rp/Rn is exceeded from 0.58<Rp/Rn<1.65, thecorrection of chromatic aberration, in particular, becomes difficult.

When there are many lenses close to each other, holding the lenses isdifficult for the conventional holding means as shown in FIG. 8. Thesurface shape of the lens is very accurately in the microscope objectiveunit using wavelengths in a deep ultraviolet range. Accordingly, holdingthe lenses is difficult for the holding means as shown in FIG. 9. Themicroscope objective unit requires high precision for a clearancebetween lenses. It is also difficult to use the holding means as shownin FIG. 10. These points represent a first problem.

In consideration of the first problem, it would be desirable to providean optical unit which has a lens frame capable of holding the set ofclosely arranged lenses and is capable of improve the accuracy ofcentering and appropriately maintaining the arrangement of the set oflenses with respect to the lens frame, that is the positionalrelationship between the set of lenses and the lens frame.

Further, it would be desirable to provide assembling an optical unitwhich has a lens frame capable of holding a set of closely arrangedlenses and is capable of improve the accuracy of centering andappropriately maintaining the arrangement of the set of lenses withrespect to the lens frame, that is the positional relationship betweenthe set of lenses and the lens frame.

Generally, when lenses are directly fixed in a lens barrel, the opticalunit makes it difficult to arrangement the lenses precisely to thebarrel. For this reason, the optical unit uses a lens-holding apparatusfor holding lenses. The lens-holding apparatus holds the lenses via alens frame as shown in FIG. 18. For example, the lens-holding apparatusis used to highly accurately hold set of lenses such as an objectiveunit. FIG. 18 shows an ordinary lens-holding apparatus.

The optical unit such as an objective unit of a microscope has aplurality of lenses. For example, the optical unit in FIG. 18 has threelenses 1101, 1102, and 1103. In this optical unit, errors on deceteringor decentration for the lenses 1101, 1102, and 1103 greatly affect theoptical characteristics. Accordingly, the lenses 1101, 1102, and 1103are assembled so as to improve accuracy of optical centers of lensframes 1111, 1112, and 1113 for holding the respective lenses. As aresult, the optical center of each lens can maintain a predeterminedaccuracy with reference to a lens barrel 1120. This can provide Opticalunit (objective unit) comprising the lens-holding apparatus withintended optical characteristics.

The above-mentioned conventional lens-holding apparatus has thefollowing problem. If there is a small clearance C between lenses alongthe optical axis in FIG. 18, the lenses interfere with each other duringassembly. Further, for example, FIG. 19 shows a structure in which alens clearance is small not only in a direction along the optical axis,but also in a direction crossing the optical axis. Such structureincreases the risk of interference between lenses. FIG. 19 shows thatthe lenses 1102 and 1103 having substantially the same radius ofcurvature are arranged apart from each other with clearance C in thethrust direction along the optical axis. Clearance B is a minimum gapbetween the lenses 1102 and 1103 along the perpendicular direction(radial direction) of the optical axis.

Generally, during a manufacturing process of the optical unit, a lensframe 1112 holding a lens 1102 is slightly but frequently moved in theradial direction (indicated by an arrow in FIG. 20) against a lens frame1113 holding a lens 1103. In FIG. 20, lenses 1102 and 1103 are coaxiallyarranged. When the lens frame 1112 is located with a predetermineddecentration (indicated by X in FIG. 21) with respect to the lens frame1113, the lenses interfere with each other at an interference portion(indicated by a broken line in FIG. 21). This interference of lenses1102 and 1103 flaws the lens surface of lenses 1102 and 1103 unlikecontact of metallic lens frames with each other. The flawed lens maycause a defect in the performance and the appearance. Especially, aconvex lens such as the lens 1103 is often formed of relatively softglass material such as CaF2 (fluorite) according to the optical design.Such the convex lens is flawed by a small amount of the interferenceeasily.

The flawed lens is defective. Even though the lens does not becomedefective, special care must be taken to handle such lens during themanufacturing process. Consequently, the productivity of such opticalunit is degraded. These points constitute a second problem.

In consideration of the above-mentioned second problem in the prior art,it would be desirable to provide an optical unit including alens-holding apparatus which prevents the lenses from being damaged byinterference of the lenses and is easily handled in the manufacturingprocess.

BRIEF SUMMARY OF THE INVENTION

An optical unit according to a first aspect of the invention comprisesat least one lens having an outer surface; at least one lens frame tohold the lens. The lens frame includes a cylindrical portion having anedge and an inner surface to contact the outer surface of the lens; anda U-shaped portion to hold the lens. The U-shaped portion has a U-shapedcross-section, and is formed integrally with the cylindrical portion atthe edge thereof; so that the lens held by the U-shaped portion isadhesively bonded to the cylindrical portion and then the U-shapedportion is removed from the cylindrical portion.

A method of assembling an optical unit according to another aspect ofthe invention is assembling the optical unit. The optical unit comprisesa lens, a cylindrical lens frame, and a cylindrical lens barrel. Thelens includes an outside diameter, an outer periphery having a centeraxis, and an optical axis coaxially arranged to the center axis. Thecylindrical lens frame holds the lens. The frame includes an outsidediameter, an outer periphery having a center axis, a cylindrical portionand a center axis. The cylindrical portion has a center axis andsubstantially the same inside diameter as the outside diameter of thelens. The center axis of the frame coaxially arranged to the center axisof the outer periphery of the cylindrical portion. The cylindrical lensbarrel includes a barrel hole having substantially the same an insidediameter as the outside diameter of the lens frame, the barrel holehaving a lens-holding surface with which the frame is to be contact. Thecylindrical portion has one opening end and the other opening end alongthe center axis. The cylindrical portion has a U-shaped portion. TheU-shaped portion has a U-shaped cross-section and a lens-retainingsurface for positioning the lens along the optical axis. The opticalunit assembling method comprises removing the U-shaped portion from thecylindrical portion after adhesively bonding the lens to the cylindricalportion, and fixing the lens frame to the lens-holding surface afterremoving the U-shaped portion from the cylindrical portion.

A method of assembling an optical unit according to yet another aspectof the invention mounts a lens frame to hold a lens in a lens barrel.The lens has optical axis. The lens frame has a lens-holding surface forholding an outer surface of a lens. The assembling method comprisesforming a U-shaped portion to the lens frame. The U-shaped portion has aU-shaped cross-section and a lens-retaining surface for positioning thelens along the optical axis.

In addition, the assembling method comprises bonding the lens on thelens frame by adhesive, removing the U-shaped portion from the lensframe, and mounting the lens frame in the lens barrel.

An optical unit according to still another aspect of the inventioncomprises a plurality of lenses, each of the lenses having a lenssurface and an optical axis, and a lens-holding apparatus. Thelens-holding apparatus includes a plurality of lens frames holding atleast one lens respectively, and a lens barrel. The lens frames haveportions overlapping with each other in a radial direction orthogonal tothe optical axis when the lens frames are arranged adjacent each otheralong the optical axis. The lens-holding apparatus is provided withfirst clearance in the adjacent two lens frames between the overlapportion of one lens frame and the overlap portion of the other lensframe in a radial direction orthogonal to the optical axis. Thelens-holding apparatus is provided with second clearance in the radialdirection between the lens surfaces of lenses held by the adjacent twolens frames. The first clearance is smaller than the second clearance.The lens frames is arranged in the lens barrel.

An optical unit according to still another aspect of the inventioncomprises the lenses, and a lens-holding apparatus. The lenses have anouter surface, a lens surface, and an optical axis. The lens-holdingapparatus includes a plurality of lens frames and a lens barrel. Thelens frames holds at least one lens respectively. The lens frameincludes a cylindrical portion and a U-shaped portion. The cylindricalportion has an edge and an inner surface to contact the outer surface ofthe lens. The U-shaped portion to holds the lens has a U-shapedcross-section, the U-shaped portion being formed integrally with thecylindrical portion at the edge thereof; so that the lens held by theU-shaped portion is adhesively bonded to the cylindrical portion andthen the U-shaped portion is removed from the cylindrical portion. Inaddition, the lens frames have portions overlapping with each other in adirection along the optical axis when the lens frames are arrangedadjacent each other along the optical axis. The lens-holding apparatusis provided with first clearance in the adjacent two lens frames betweenthe overlap portion of one lens frame and the overlap portion of theother lens frame in a radial direction orthogonal to the optical axis.The lens-holding apparatus is provided with second clearance in theradial direction between the lens surfaces of lenses held by theadjacent two lens frames. The first clearance is smaller than the secondclearance. The lens frames is arranged in the lens barrel.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a partially side view showing an objective unit according to afirst embodiment;

FIG. 2 is a vertical sectional view showing a set of lenses held in alens frame in FIG. 1;

FIG. 3 is a vertical sectional view showing a lens assembly beforeremoving a U-shaped portion according to the first embodiment;

FIG. 4 is a vertical sectional view showing a lens assembly of anobjective unit according to a second embodiment;

FIG. 5 shows an optical system for the objective unit in FIG. 4;

FIG. 6 is a vertical sectional view showing a lens assembly beforeremoving a U-shaped portion according to the second embodiment;

FIG. 7 is a vertical sectional view showing a conventional objectiveunit;

FIG. 8 is a sectional view showing a conventional lens frame holding acemented lens;

FIG. 9 is a sectional view showing a conventional lens frame holdinglenses by means of caulking;

FIG. 10 shows a conventional lens assembly having a clearance ring;

FIG. 11 is a sectional view showing an optical system which isdifficultly held by the conventional lens frame;

FIG. 12 is a vertical sectional view showing a lens assembly formed byintegrating the lens frame and the clearance ring in FIG. 10;

FIG. 13 is a sectional view showing a 2-group configuration lens-holdingapparatus according to a third embodiment;

FIG. 14 is an explanatory diagram illustrating the amount ofmisalignment between a front group lens frame and a rear group lensframe of the lens-holding apparatus according to the third embodiment;

FIG. 15 is a sectional view showing a 3-group configuration lens-holdingapparatus according to a fourth embodiment;

FIG. 16 is a sectional view showing a 2-group configuration lens-holdingapparatus according to a fifth embodiment;

FIG. 17 is a sectional view showing a 2-group configuration lens-holdingapparatus according to a sixth embodiment;

FIG. 18 is a sectional view showing a 3-group configuration lens-holdingapparatus according to the prior art;

FIG. 19 is an explanatory diagram illustrating an assembling work for3-group configuration lens-holding apparatus according to the prior art;

FIG. 20 is an explanatory diagram illustrating a problem of the priorart; and

FIG. 21 is an explanatory diagram illustrating a problem of the priorart.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described in further detailwith reference to the accompanying drawings.

First Embodiment

FIGS. 1 through 3 show the first embodiment. FIG. 1 is a partially sideview showing an objective unit according to the first embodiment. FIG. 2is an enlarged vertical sectional view showing a set of lenses held in alens frame. FIG. 3 is a vertical sectional view showing a lens assemblybefore removing a U-shaped portion.

In FIG. 1, the objective unit of a microscope has a plurality of lenses1, a plurality of lens frames 2 for holding the respective lenses, and alens barrel 3 for holding the plurality of lens frames 2.

Each lens 1 has an outside diameter and outer periphery. The outerperiphery has center axis. Each lens frame 2 is cylindrical and isprovided with a cylindrical portion for holding each lens 1. Thecylindrical portion has an inside diameter substantially correspondingto the outside diameter of each lens 1. The plurality of lens frames 2has substantially the same outside diameter. In each lens frame 2, thecenter axis of the cylindrical portion substantially coaxially arrangedto the center axis of an outer periphery thereof.

The lens barrel 3 is cylindrical and is provided with a barrel hole forholding the plurality of lens frames 2. The barrel hole has an insidediameter substantially corresponding to the outside diameter of eachlens frame 2.

FIG. 2 shows a set of lenses 1A provided by combining the plurality oflenses 1. More specifically, the set of lenses 1A includes a convex lens11, a concave lens 12, and a convex lens 13 in the plurality of lenses1. The set of lenses 1A has a close clearance between the lenses.

As shown in FIG. 2, the lens frames 21, 22, and 23 hold the convex lens11, the concave lens 12, and the convex lens 13, respectively. The lensframes 21 and 23 are configured like a known lens frame. Specifically,the lens frames 21 and 23 each have a lens frame edge orthogonal to thecenter axis. The lens frame edge is formed with high precision along adirection orthogonal to the center axis. Each of the lens frames 21 and23 has a stop which projects toward the inside of the frame. The stophas a retaining surface for seating a lens. The configuration of thelens frame 22 will now be described with reference to FIG. 3.

The following describes a method of mounting lenses in the lens frames21, 22, and 23. A conventional lens holding means is used for the convexlenses 11 and 13. Specifically, the convex lenses 11 and 13 are droppedinto the lens frames 21 and 23. The lens frames 21 and 23 use theretaining surfaces to hold the convex lenses 11 and 13. After centeringto be described later, the convex lenses 11 and 13 are adhesively bondedto the inner surface of the cylindrical portion.

When the convex lenses 11 and 13 are bonded to the lens frames 21 and23, the lens frames 21 and 23 move along the lens frame edges. Due tothis moving, optical axes of the convex lenses 11 and 13 move along thelens frame edges. For this reason, the convex lenses 11 and 13 arecentered so that their optical axes coaxially are arranged to the centeraxes of the lens frames 21 and 22. After this centering, the externalsurfaces of the convex lenses 11 and 13 are adhesively bonded to theinner surfaces of the lens frames 21 and 23.

The concave lens 12 is adhesively bonded to the lens frame 22. The lensframe 22 is formed by processing a lens frame blank 22′ as shown in FIG.3. The lens frame blank 22′ has a U-shaped portion 22 a for holding alens. The U-shaped portion 22 a has a U-shaped cross-section.

In order to fix the concave lens 12 to the lens frame 22, the concavelens 12 is first dropped into the lens frame blank 22′. Then, theconcave lens 12 is centered and is adhesively bonded to the lens frameblank 22′. After the concave lens 12 is fixed, the U-shaped portion 22 ais removed from the blank 22′ by a cutter 25 (cutting tool, millingcutter, etc.) as shown in FIG. 3. The lens frame 22 is formed in thismanner.

The lens frame blank 22′ has substantially the same outside diameter asthat of the lens frames 21 and 23. Namely, the lens frame blank 22′ hassubstantially the same outside diameter as the dimension (finisheddimension) of the lens frame 22 after the processing.

The lens frame blank 22′ has an outer periphery. The outer periphery hasa center axis. The lens frame blank 22′ has a center axis. The centeraxis of the blank 22′ is the center axis of the periphery thereof. Thelens frame blank 22′ has edges 22 b and 22 c extending in a directionorthogonal to the center axis. The edges 22 b and 22 c are spaced with apredetermined distance along the center axis. The spaced distance issubstantially coincident with a dimension along the center axis of thelens frame 22 after the U-shaped portion 22 a is removed from the blank22′. Namely, the width dimension from the edges 22 b to 22 c of the lensframe blank 22′ is finished to be substantially the same as the finisheddimension of lens frame 22.

The lens frame blank 22′ has a cylindrical portion. The cylindricalportion has substantially the same inside diameter as the outsidediameter of the concave lens 12. Namely, the inside diameter of thecylindrical portion is finished to be the same as the finisheddimension. Accordingly, the concave lens 12 can be arranged in thecylindrical portion. The outer surface of the concave lens 12 is incontact with the inner surface 22 e. The outer surface of the concavelens 12 is adhesively bonded to an inner surface 22 e of the cylindricalportion. Thus, the inner surface 22 e of cylindrical portion works asthe lens-holding surface for holding the concave lens 12.

The cylindrical portion has a center axis. The center axis coaxiallyarranged to the center axis of inside periphery thereof. The center axisof the cylindrical portion substantially coaxially arranged to thecenter axis of the lens frame 22′. The cylindrical portion has one andthe other ends in a direction along the center axis of the lens frameblank 22′. One end is positioned to the side of an edge 22 c in adirection along the center axis of the lens frame blank 22′. The otherend is positioned to the side of an edge 22 b in a direction along thecenter axis of the lens frame blank 22′. Each of the both ends has anopening.

The U-shaped portion 22 a is integrally formed with the lens frame blank22′ at edge 22 b. Specifically, the U-shaped portion 22 a is locatednear the inner surface 22 e of the cylindrical portion in a directionorthogonal to the center axis of the lens frame blank 22′. Namely, theU-shaped portion 22 a is located near the opening of the cylindricalportion on the edge 22 b. The U-shaped portion 22 a projects toward theinside of the cylindrical portion. Further, the U-shaped portion 22 aprojects toward the inside of the cylindrical portion in a directionalong the center axis of the lens frame blank 22′. Namely, the U-shapedportion 22 a has a top portion inserted in the cylindrical portion. Thetip surface of the top portion is spaced for a predetermined distancefrom the edge 22 b along the center axis of the lens frame blank 22′.Consequently, the U-shaped portion 22 a has a surface opposite the innersurface 22 e of the cylindrical portion. The U-shaped portion 22 asupports the lens by using the tip surface of the top portion and thesurface opposite the inner surface 22 e. Accordingly, the tip surface ofthe top portion and the surface opposite the inner surface 22 e providea lens-retaining surface. The lens-retaining surface is represented bythe reference numeral 22 f in FIG. 3.

The distance from the edge 22 c to the retaining surface 22 f isdetermined according to a position for mounting the concave lens 12.Specifically, the distance from the edge 22 c to the retaining surface22 f is determined in a direction along the center axis of thecylindrical portion so that the concave lens 12 can be arranged at apredetermined position with reference to the convex lenses 11 and 13.The distance from the edge 22 c to the retaining surface 22 f isfinished to be a dimension so that the concave lens 12 in FIG. 2 canensure a predetermined clearance between the convex lenses 11 and 13when the lens frame 22 is mounted in the lens barrel 3 in FIG. 1.

As shown in FIG. 3, the U-shaped portion 22 a has a space 22 g between alens-retaining surface 22 f and the inner surface 22 e. In other words,the space 22 g is provided between the inner surface 22 e and the topportion of the U-shaped portion 22 a. As shown in FIG. 3, the space 22 gis provided from the edge 22 b toward the edge 22 c along the centeraxis of the cylindrical portion. When the concave lens 12 is held by thelens-retaining surface 22 f, the concave lens 12 is separated from theedge 22 b due to the space 22 g. The space 22 g can prevent the concavelens 12 from interfering with the tip of the cutter 25. Accordingly, itis possible to say that the space 22 g is formed between the retainingsurface 22 f and the inner surface 22 e in such a degree as to preventthe tip of the cutter 25 from interfering with the concave lens 12.

The lens frame 22 configures the lens assembly by fixing the concavelens 12 as mentioned above. The lens frames 21 and 23 also configure alens assembly holding the convex lenses 11 and 13. As shown in FIG. 1,these lens assemblies are arranged in the lens barrel 3 and configurethe objective unit together with the other members.

According to the embodiment, the lens frame can hold lenses even in theobjective unit having an optical system which makes it difficult to holdlenses according to the conventional holding method due to closelyarranged lenses. Consequently, the lenses can be highly accuratelypositioned with reference to the lens frames. In addition, the lensescan be highly accurately centered with reference to the lens frames.Further, each lens can be highly accurately maintained with apredetermined clearance in relation to adjacent lenses. Accordingly, theembodiment can provide a highly accurate objective unit.

Second Embodiment

FIGS. 4 through 6 show the second embodiment. FIG. 4 is a verticalsectional view showing a lens assembly of an objective unit. FIG. 5shows an optical system for the objective unit in FIG. 4. FIG. 6 is avertical sectional view showing a lens assembly before removing aU-shaped portion.

The objective unit in FIG. 4 has two lens assemblies. One lens assemblyhas a set of lenses A and a lens frame 47 for holding the set of lensesA. The set of lenses A has the convex lens 31 and the concave lens 32.The convex lens 31 and the concave lens 32 are bonded to each other. Theconvex lens 31 and the concave lens 32 configure a cemented lens. Theother lens assembly has a set of lenses B and a lens frame 46 forholding the set of lenses B. FIG. 5 shows the optical system byextracting only the lens portion. The optical system is the same as thatcomprising the set of lenses A and the set of lenses B according to theabove-mentioned prior art shown in FIG. 10. In the embodiment, the lensframe 47 differs from the conventional lens frame. The lens frame 47 isshown in FIG. 4. The other lens assembly having the set of lenses B andthe lens frame 46 is the same as the prior art.

As shown in FIG. 4, the set of lenses A is adhesively bonded to the lensframe 47. During the bonding, the set of lenses A is dropped into thelens frame blank 47′ as shown in FIG. 6. Like the first embodiment, thelens frame blank 47′ is the lens frame 47 before processed. The lensframe blank 47′ has a U-shaped portion 47 a. The lens frame blank 47′ isfinished in the same manner as the first embodiment. Specifically, thelens frame blank 47′ has the same finished dimensions as the lens frame47 concerning the width dimension from the edges 47 b to 47 c, thediameter of an outer surface 47 d, and the diameter of an inner surface47 e as the lens holding surface.

The U-shaped portion 47 a is projected from the inner surface 47 e ofthe edge 47 b. Like the first embodiment, the U-shaped portion 47 a hasa retaining surface 47 f as the lens-retaining surface for the concavelens 32. The U-shaped portion 47 a forms a space 47 g between theretaining surface 47 f and the inner surface 47 e in such a degree as toprevent the tip of the cutter 25 (see FIG. 3) from interfering with theconcave lens 32.

The distance from the edge 47 c to the retaining surface 47 f isdetermined according to the position of mounting the concave lens 32.Specifically, the distance from the edge 47 c to the retaining surface47 f is determined in a direction along the center axis of thecylindrical portion so that the concave lens 32 can be arranged at apredetermined position with reference to the convex lens 33 (see FIGS. 4and 5). The distance from the edge 47 c to the retaining surface 47 f isset to be a dimension so that the concave lens 32 can ensure apredetermined clearance against the convex lens 33 when the lens frame47 is mounted in a lens barrel (not shown). The U-shaped portion 47 a isremovable. Hence, the lens frame 47, when mounted in the lens barrel,can be arranged at the side of the lens frame 46 with respect to lens31. When the lens frame 47 is formed relatively long along its centeraxis, it is possible to easily drop the set of lenses A onto thelens-retaining surface. Accordingly, the lens frame 47 is capable ofeasily dropping the set of lenses A and providing a relatively longclearance against the lens frame 46.

The set of lenses A is dropped into the lens frame blank 47′ and iscentered. The set of lenses A is then adhesively bonded to the lensframe blank 47′. The cutter 25 is used to cut and remove the U-shapedportion 47 a from the lens frame blank 47′. This removing forms a slope47 h as shown in FIG. 4 on the lens frame 47. The lens frame 47configures a lens assembly having the set of lenses A. The lens frame 46also configures a lens assembly holding the set of lenses B. These lensassemblies are arranged in the lens barrel 3 (not shown) and configuresthe objective unit together with the other members.

In addition to the same effects as for the first embodiment, the secondembodiment can provide a clearance between adjacent lens frames withoutusing a clearance ring according to the prior art. Consequently, theembodiment can decrease the number of lens frames and provide anobjective unit satisfactory for the accuracy.

The above-mentioned optical unit can provide a lens frame capable ofholding a group of adjacent lenses. The optical unit can provideaccuracy of the centering, and can appropriately ensure positionalrelationship between a group of lenses and the lens frame.

According to above-mentioned embodiments, a lens is dropped in theU-shaped portion of the lens frame and then is centered. An adhesive isapplied between the lens frame and the lens and is hardened. Then, acutter is used to cut and remove the U-shaped portion. The small-sizedlens frame with the lens is mounted in the lens barrel. Accordingly, thelens frame can hold a group of adjacent lenses. It is possible toprovide an optical unit which improves accuracy of the centering andappropriately ensures positional relationship between the group oflenses and the lens frame.

According above-mentioned, when the U-shaped portion is removed from thelens frame blank, the tip of the cutter is stopped at a space providedbetween the lens holding surface and the lens-retaining surface.Accordingly, it is possible to remove the angled U-shaped portion fromthe lens frame blank without flawing the lens surface with the cutter.

Third Embodiment

FIG. 13 is a sectional view showing a 2-group configuration lens-holdingapparatus. FIG. 14 is an explanatory diagram illustrating the amount ofmisalignment between a front group lens frame and a rear group lensframe of the lens-holding apparatus.

The lens-holding apparatus in FIG. 13 has a lens barrel 210, a frontgroup lens frame 211 for holding a front group lens 201, and a reargroup lens frame 212 for holding a rear group lens 202.

In FIG. 13, the front group lens 201 and the rear group lens 202 haveapproximate radiuses of curvature for their lens surfaces adjacent toeach other. For example, the front group lens 201 has radius ofcurvature Rp of 10 mm for the surface with the positive power. The reargroup lens 202 has radius of curvature Rn of 10.5 mm for the surfacewith the negative power.

As lens frames for holding the optical system, the front group lensframe 211 and the rear group lens frame 212 have substantially the sameoutside diameter. The front group lens frame 211 and the rear group lensframe 212 have outer peripheries respectively. Each of the outerperipheries has a center axis. The front group lens frame 211 and therear group lens frame 212 have center axes. Each of the center axes ofthe front group lens frame 211 and the rear group lens frame 212 is thecenter axis of the periphery thereof respectively. The front group lensframe 211 holds the front group lens 201 so that the center axis of theframe substantially is coaxially arranged to the optical axis of thefront group lens 201. Likewise, the rear group lens frame 212 holds therear group lens 202 so that the center axis of the frame substantiallycoaxially arranged to the optical axis of the rear group lens 202. Thefront group lens frame 211 and the rear group lens frame 212 areinserted in the lens barrel 210 and are arranged to a predeterminedposition. When the front group lens frame 211 and the rear group lensframe 212 are arranged to the predetermined position, the center axis ofthe front group lens frame 211 substantially coaxially arranged to thatof the rear group lens frame 212.

When the front group lens frame 211 and the rear group lens frame 212are arranged to the predetermined position, there is provided apredetermined clearance, i.e., a thrust clearance C1 between the frontgroup lens 201 and the rear group lens 202 along a direction of theoptical axes. For example, the thrust clearance C1 is 0.5 mm.

At the predetermined position, the front group lens frame 211 and therear group lens frame 212 have overlap portions 211 a and 212 a whichoverlap with each other along the center axes. In other words, the frontgroup lens frame 211 and the rear group lens frame 212 have theoverlapping portions which overlap with each other along the opticalaxes of the front group lens 1 and the rear group lens 2. The overlapportions 211 a and 212 a overlap with each other also in the radialdirection.

When the front group lens frame 211 and the rear group lens frame 212are arranged to the predetermined position, the overlap portions areseparated from each other by clearance A1 in the radial direction. Thefront group lens 201 and the rear group lens 202 are separated from eachother by thrust clearance C1 along the optical axis direction asmentioned above. The front group lens 201 and the rear group lens 202are separated from each other by clearance B1 in the radial direction.The front group lens frame 211 and the rear group lens frame 212 areconfigured so that the clearance A1 is smaller than the clearance B1.

The following describes actions in the assembling of the lens-holdingapparatus according to the above-mentioned configuration. As shown inFIG. 13, the front group lens frame 211 and the rear group lens frame212 provide the clearance A1 in the radial direction. In this case,before the front group lens frame 211 and the rear group lens frame 212are mounted in the lens barrel 210, the front group lens frame 211 ismovable in the radial direction for a maximum of the clearance A1 withreference to the rear group lens frame 212 as shown in FIG. 14. At thesame time, the front group lens frame 211 is prevented from movingbeyond the clearance A1 in the radial direction with reference to therear group lens frame 212. In this case, as shown in FIG. 13, there isan allowance of dimension (=clearance B1−clearance A1) in the radialdirection between the front group lens 201 and the rear group lens 202.Accordingly, the lenses do not interfere with each other.

Since the embodiment assumes the radius of curvature Rp=10 (mm), theradius of curvature Rn=10.5 (mm), and the thrust clearance C1=0.5 (mm),Rp/Rn becomes 0.952. As indicated by the above-mentioned values for theradiuses of curvature Rp and Rn, the radius of curvature of the convexlens is smaller than that of the concave lens on the adjacent lenssurfaces. The above-mentioned value for Rp/Rn satisfies the condition of0.58<Rp/Rn<1.65 in claim 7. Generally, adjacent lens surfaces veryeasily interfere with each other when a value for Rp/Rn ranges fromapproximately 0.58 to 1.65 as mentioned above. Since the lens holdingmember according to the embodiment provides the predetermined clearanceA1, the adjacent lens surfaces can be prevented against interferencewith each other.

According to the embodiment, the overlap portion restricts the amount ofmisalignment in the radial direction between the front group lens frame211 and the rear group lens frame 212. Accordingly, it is possible toprevent occurrence of a flaw due to interference of adjacent lenseshaving approximate radiuses of curvature and a small thrust clearancetherebetween. The lens-holding apparatus facilitate operations duringthe manufacturing process.

Fourth Embodiment

FIG. 15 shows the fourth embodiment. FIG. 15 is a sectional view showinga 3-group configuration lens-holding apparatus. The lens-holdingapparatus in FIG. 15 has a lens barrel 210, a first group lens frame 231for holding a first group lens 221, a second group lens frame 232 forholding a second group lens 222, and a third group lens frame 233 forholding a third group lens 223.

The first group lens 221, the second group lens 222, and the third grouplens 223 have such radiuses as to approximate radiuses of curvature forthe adjacent lens surfaces. As shown in FIG. 15, there is provided apredetermined clearance, i.e., a thrust clearance C2 between the secondgroup lens 222 and the third group lens 223 along their optical axeswhen these match. The thrust clearance C2 is relatively small.

As lens frames for holding the optical system, the first group lensframe 231, the second group lens frame 232, and the third group lensframe 233 have substantially the same outside diameter. The front grouplens frame 211 and the rear group lens frame 212 have outer peripheriesrespectively. Each of the outer peripheries has a center axis. The frontgroup lens frame 211 and the rear group lens frame 212 have center axes.Each of the center axes of the frame 211, 212 is the center axis of theouter peripheries thereof respectively.

As lens frames for holding the optical system, the first group lensframe 231, the second group lens frame 232, and the third group lensframe 233 are fit into a lens barrel 230. Like the first embodiment, thefirst group lens frame 231, the second group lens frame 232, and thethird group lens frame 233 have overlap portions 231 a, 232 a, 232 b,and 233 a which overlap with each other along the center axes. Theoverlap portions 231 a, 232 a, 232 b, and 233 a overlap with each otheralso in the radial direction.

There is a clearance A2 between the overlap portions 232 b and 233 a ofthe second group lens frame 232 and the third group lens frame 233 inthe radial direction. Like the first embodiment, there is a thrustclearance C2 between the second group lens 22 and the third group lens23. The second group lens 22 and the third group lens 23 provide aclearance B2 in the radial direction of the lenses corresponding to thethrust clearance C2. At this time, the clearance A2 between the secondgroup lens frame 232 and the third group lens frame 233 is defined sothat A2 becomes smaller than B2.

Since actions in the assembling of the above-mentioned 3-groupconfiguration lens-holding apparatus are the same as those for the thirdembodiment, a description is omitted.

According to the fourth embodiment, the 3-group configuration canprovide the same effects as for the third embodiment. This completelyapplies to many lens-holding apparatuses comprising four groups or more.It just needs to ensure the above-mentioned structural dimensions foradjacent lens surfaces.

As shown in the first embodiment, the first group lens frame 231, thesecond group lens frame 232, and the third group lens frame 233according to the fourth embodiment can be finished after being formed inthe lens frame blank in the middle of the process. Accordingly, thefirst group lens frame 231, the second group lens frame 232, and thethird group lens frame 233 according to the embodiment can be assembledin the optical unit like the first embodiment.

Fifth Embodiment

FIG. 16 is a sectional view showing a 2-group configuration lens-holdingapparatus according to the fifth embodiment. Like the third embodiment,the lens-holding apparatus in FIG. 16 has a lens barrel 250, a frontgroup lens frame 251 for holding a front group lens 241, and a reargroup lens frame 252 for holding a front group lens 242. In FIG. 16, thefront group lens 241 and the front group lens 242 have approximateradiuses of curvature for their lens surfaces adjacent to each other.For example, the front group lens 241 has radius of curvature Rp1 of 3.0mm for the surface with the positive power. The front group lens 242 hasradius of curvature Rn1 of 4.1 mm for the surface with the negativepower.

The lens holding means according to the embodiment is the same as forthe third embodiment. When the front group lens frame 251 and the reargroup lens frame 252 are arranged to the predetermined position, thereis provided a predetermined clearance, i.e., a thrust clearance C3between the front group lens 241 and the front group lens 242 along theoptical axes of themselves. For example, the thrust clearance C3 is 0.5mm.

At the predetermined position, the front group lens frame 251 and therear group lens frame 252 according to the embodiment, like the thirdembodiment, have overlap portions 251 a and 252 a which overlap witheach other along the center axes. In other words, the front group lensframe 251 and the rear group lens frame 252 have the overlappingportions which overlap with each other along the optical axes of thefront group lens 241 and the rear group lens 242. The overlap portions251 a and 252 a overlap with each other also in the radial direction.

According to the fifth embodiment, like the third embodiment, theoverlap portions are separated from each other by clearance A3 in theradial direction. According to the fifth embodiment, like the thirdembodiment, the front group lens 241 and the front group lens 242 areseparated from each other by thrust clearance C3 along the optical axesof themselves as mentioned above. The front group lens 241 and the frontgroup lens 242 are separated from each other by clearance B3 in theradial direction. At this time, the front group lens frame 251 and therear group lens frame 252 are configured so that the clearance A3 issmaller than the clearance B3.

The following describes actions in the assembling of the lens-holdingapparatus according to the above-mentioned configuration. As shown inFIG. 16, the front group lens frame 251 and the rear group lens frame252 provide the clearance A3 in the radial direction. In this case,before the front group lens frame 251 and the rear group lens frame 252are mounted in the lens barrel 250, the front group lens frame 251 ismovable in the radial direction for a maximum of the clearance A3 withreference to the rear group lens frame 252. At the same time, the frontgroup lens frame 251 is prevented from moving beyond the clearance A3 inthe radial direction with reference to the rear group lens frame 252. Inthis case, as shown in FIG. 13, there is an allowance of dimension(=clearance B3−clearance A3) in the radial direction between the frontgroup lens 241 and the front group lens 242. Accordingly, the lenses donot interfere with each other.

Since the embodiment assumes the radius of curvature Rp1=4.1 (mm), theradius of curvature Rn1=3.0 (mm), and the thrust clearance C3=0.5 (mm),Rp1/Rn1 becomes 1.366. As indicated by the above-mentioned values forthe radiuses of curvature Rp1 and Rn1, the radius of curvature of theconvex lens is greater than that of the concave lens on the adjacentlens surfaces. A value for Rp1/Rn1 ranges from 0.58 to 1.65.Accordingly, adjacent lens surfaces very easily interfere with eachother. More specifically, the value for Rp1/Rn1 indicates a criticalvalue for the upper bound under a condition susceptible to interferenceof the lenses with each other. Since the lens holding member accordingto the embodiment provides the predetermined clearance A3, the adjacentlens surfaces can be prevented against interference with each other.

Sixth Embodiment

FIG. 17 is a configuration diagram of a 2-group configurationlens-holding apparatus according to a sixth embodiment. Like the thirdembodiment, the lens-holding apparatus in FIG. 17 has a lens barrel 270,a front group lens frame 271 for holding a front group lens 261, and arear group lens frame 272 for holding a front group lens 262. In FIG.17, the front group lens 261 and the front group lens 262 haveapproximate radiuses of curvature for their lens surfaces adjacent toeach other. For example, the front group lens 261 has radius ofcurvature Rp2 of 7.8 mm for the surface with the positive power. Thefront group lens 262 has radius of curvature Rn2 of 8.6 mm for thesurface with the negative power.

The lens holding means according to the embodiment is the same as forthe third embodiment. When the front group lens frame 271 and the reargroup lens frame 272 are arranged to the predetermined position, thereis provided a predetermined clearance, i.e., a thrust clearance C4between the front group lens 261 and the front group lens 262 along eachoptical axis direction. For example, the thrust clearance C4 is 0.1 mm.

At the predetermined position, the front group lens frame 271 and therear group lens frame 272 according to the embodiment, like the thirdembodiment, have overlap portions 271 a and 272 a which overlap witheach other along the center axes. In other words, the front group lensframe 271 and the rear group lens frame 272 have the overlappingportions which overlap with each other along the optical axes of thefront group lens 261 and the rear group lens 262. The overlap portions271 a and 272 a overlap with each other in the radial direction.

According to the sixth embodiment, like the third embodiment, theoverlap portions are separated from each other by clearance A4 in theradial direction. According to the sixth embodiment, like the thirdembodiment, the front group lens 261 and the front group lens 262 areseparated from each other by thrust clearance C4 along the optical axisdirection as mentioned above. The front group lens 261 and the frontgroup lens 262 are separated from each other by clearance B4 in theradial direction. At this time, the front group lens frame 271 and therear group lens frame 272 are configured so that the clearance A4 issmaller than the clearance B4.

The following describes actions in the assembling of the lens-holdingapparatus according to the above-mentioned configuration. As shown inFIG. 17, the front group lens frame 271 and the rear group lens frame272 provide the clearance A4 in the radial direction. In this case,before the front group lens frame 271 and the rear group lens frame 272are mounted in the lens barrel 270, the front group lens frame 271 ismovable in the radial direction for a maximum of the clearance A4 withreference to the rear group lens frame 272. At the same time, the frontgroup lens frame 271 is prevented from moving beyond the clearance A4 inthe radial direction with reference to the rear group lens frame 272. Inthis case, as shown in FIG. 17, there is an allowance of dimension(=clearance B4−clearance A4) in the radial direction between the frontgroup lens 261 and the front group lens 262. Accordingly, the lenses donot interfere with each other.

Since the embodiment assumes the radius of curvature Rp2=7.8 (mm), theradius of curvature Rn2=8.6 (mm), and the thrust clearance C4=0.1 (mm),Rp2/Rn2 becomes 0.906. As indicated by the above-mentioned values forthe radiuses of curvature Rp2 and Rn2, the radius of curvature of theconvex lens is smaller than that of the concave lens on the adjacentlens surfaces. A value for Rp2/Rn2 ranges from 0.58 to 1.65.Accordingly, adjacent lens surfaces very easily interfere with eachother. More specifically, the value for Rp2/Rn2 indicates a criticalvalue for the lower bound under a condition susceptible to interferenceof the lenses with each other. Since the lens holding member accordingto the embodiment provides the predetermined clearance A4, however, theadjacent lens surfaces can be prevented against interference with eachother.

The sixth embodiment can provide the same effects as for the thirdembodiment also to the lens-holding apparatus having the optical systemunder a condition susceptible to interference of the lenses with eachother, wherein the value for Rp2/Rn2 (0.906) indicates a critical valuefor the lower bound.

With reference to the third and sixth embodiments, there have beendescribed the measures for preventing interference between adjacentlenses having approximate radiuses of curvature and a small thrustclearance. An example of such lens system is a microscope objective lensusing wavelengths in a deep ultraviolet range. The microscope objectivelens for the deep ultraviolet range uses more lenses than a microscopeobjective lens using wavelengths in a visible range. The ultravioletlight changes the quality of an adhesive bonding the lenses to eachother, causing the possibility of degrading the transmittance. It isdesirable not to use a cemented lens. To eliminate the use of a cementedlens, it is necessary to very closely arrange a convex lens and aconcave lens having approximate radiuses of curvature. The shared accessaccording to the present invention can be appropriately used for suchmicroscope objective lenses for the deep ultraviolet range.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. An optical unit comprising: at least one lenshaving an outer surface; and at least one lens frame to hold the lens,the lens frame including a cylindrical portion having an edge and aninner surface to contact the outer surface of the lens, and a U-shapedportion to hold the lens, the U-shaped portion having a U-shapedcross-section, the U-shaped portion being formed integrally with thecylindrical portion at the edge thereof; so that the lens held by theU-shaped portion is adhesively bonded to the cylindrical portion andthen the U-shaped portion is removed from the cylindrical portion. 2.The optical unit according to claim 1, wherein the optical unit is amicroscope objective unit.
 3. An assembling method of an optical unit,the optical unit comprising: a lens including an outside diameter, anouter periphery having a center axis, and an optical axis coaxiallyarranged to the center axis; a cylindrical lens frame to hold the lens,the frame including an outside diameter, an outer periphery having acenter axis, a cylindrical portion having a center axis andsubstantially the same inside diameter as the outside diameter of thelens, and a center axis coaxially arranged to the center axis of theouter periphery of the cylindrical portion; and a cylindrical lensbarrel including a barrel hole having substantially the same an insidediameter as the outside diameter of the lens frame, the barrel holehaving a lens-holding surface with which the frame is to be contact, thecylindrical portion having one opening end and the other opening endalong the center axis, and having a U-shaped portion, the U-shapedportion having a U-shaped cross-section and a lens-retaining surface forpositioning the lens along the optical axis, the optical unit assemblingmethod comprising removing the U-shaped portion from the cylindricalportion after adhesively bonding the lens to the cylindrical portion,and fixing the lens frame to the lens-holding surface after removing theU-shaped portion from the cylindrical portion.
 4. The optical unitassembling method according to claim 3, wherein the U-shaped portion hasa space between the lens-holding surface and the lens-retaining surface,the space which prevents a cutter used for removing the U-shaped portionfrom interfering with the lens.
 5. An objective unit assembling methodof mounting a lens frame to hold a lens in a lens barrel, the lenshaving an optical axis, the lens frame having a lens-holding surface forholding an outer surface of a lens; the assembling method comprisingforming a U-shaped portion to the lens frame, the U-shaped portionhaving a U-shaped cross-section and a lens-retaining surface forpositioning the lens along the optical axis; bonding the lens on thelens frame by adhesive; removing the U-shaped portion from the lensframe; mounting the lens frame in the lens barrel.
 6. An optical unitcomprising: lenses having an outer surface, a lens surface, and anoptical axis; a lens-holding apparatus including a plurality of lensframes holding at least one lens respectively, and a lens barrel, thelens frame including a cylindrical portion having an edge and an innersurface to contact the outer surface of the lens, and a U-shaped portionto hold the lens, the U-shaped portion having a U-shaped cross-section,the U-shaped portion being formed integrally with the cylindricalportion at the edge thereof; so that the lens held by the U-shapedportion is adhesively bonded to the cylindrical portion and then theU-shaped portion is removed from the cylindrical portion. the lensframes having portions overlapping with each other in a direction alongthe optical axis when the lens frames are arranged adjacent each otheralong the optical axis, the lens-holding apparatus being provided withfirst clearance in the adjacent two lens frames between the overlapportion of one lens frame and the overlap portion of the other lensframe in a radial direction orthogonal to the optical axis, thelens-holding apparatus being provided with second clearance in theradial direction between the lens surfaces of lenses held by theadjacent two lens frames, the first clearance being smaller than thesecond clearance, the lens frames being arranged in the lens barrel.